The default single-container Docker instance for Mattermost is designed
for product evaluation, and sets SendEmailNotifications=false so the
product can function without enabling email. To see the product’s full
functionality, enabling SMTP email is recommended.

To enable email, configure an SMTP email service as follows:

Set up an SMTP email sending service (if you don’t yet have an
SMTP service with credentials)

If you have issues with your SMTP install, from your Mattermost team
site go to the main menu and open System Console -> Logs to look for
error messages related to your setup. You can do a search for the error
code to narrow down the issue. Sometimes ISPs require nuanced setups for
SMTP and error codes can hint at how to make the proper adjustments.